An optical fiber, after it is drawn from a preform, is coated typically with at least one polymeric coating. These coatings significantly enhance the mechanical and optical properties of the fiber. However, polymeric coatings are generally permeable to environmental elements, such as water and hydrogen, which are deleterious to the fiber. The interaction of water with the surface of a silica fiber produces surface modifications which can reduce the strength of the fiber. Also, over a period of time, hydrogen can diffuse into an optical fiber and increase the optical loss in a signal carried by that optical fiber.
In order to prevent deleterious environmental elements from interacting with the fiber, a coating which acts as an impenetrable hermetic barrier to the environment is applied between the fiber and the polymeric coating. One such coating is a carbon coating applied to the outer surface of a silica cladding of the fiber. By inducing decomposition of a suitable carbon containing organic precursor gas, e.g., acetylene, a thin carbon film is formed on the surface of the fiber, for example, as described by F. V. DiMarcello et al., in U.S. Pat. No. 5,000,541 issued Mar. 19, 1991, which is incorporated herein by reference. For optimum results, the carbon coating must be applied in a particular thickness within close tolerances. If the coating is too thin, e.g., thinner than 200 .ANG., it does not sufficiently limit the penetration of the undesirable environmental elements. On the other hand, if it is too thick, e.g., greater than 2000 .ANG., fiber strength can be reduced by microcracks which can form in the carbon coating when the fiber is under high tensile force.
An example of a dynamic manufacturing method to measure and control the thickness of a hermetic coating being applied to an unjacketed optical fiber is disclosed in U.S. Pat. No. 5,013,130, issued to R. M. Atkins et al. on May 7, 1991, and in U.S. Pat. No. 5,057,781 issued to R. M. Atkins et al. on Oct. 15, 1991, each of which is incorporated herein by reference. This manufacturing method includes the steps of depositing a hermetic coating, e.g. carbon, on a moving optical fiber being drawn from a heated preform, and measuring contactlessly the thickness of the coating deposited on the optical fiber.
In FIG. 3 is shown a schematic representation of an exemplary prior art equipment for drawing an optical fiber, 20, from a preform, 24, coating fiber 20 first with a hermetic carbon coating and then with a polymer jacket, and finally winding the jacketed fiber on a take-up reel, 49. Fiber 20 is drawn from an end of preform 24 which is heated in a furnace, 25, to its melting or softening temperature. The fiber is drawn at a controlled steady temperature and velocity for producing fibers with uniform diameter. The drawn fiber 20 moves through a diameter gauge, 26, which produces a signal representing the diameter of the fiber. This signal is forwarded from gauge 26 via a lead, 27, to a detection, analysis and feedback processor, 28. The processor converts such a measurement signal into an analogous control signal which is used via a lead, 29, for adjusting the temperature of furnace 25, and via a lead, 54, to a capstan drive control, 55, for adjusting the drawing speed, as needed. A certain minimum fiber temperature is needed for deposition of a carbon coating on the surface of the fiber. Therefore, fiber 20, drawn from preform 24, moves through an optional heater, 30, for supplementing the residual heat in fiber 20, as needed, and through thermometer or pyrometer 31, for monitoring the temperature of the fiber 20 prior to a carbon-coating stage. A signal representing the temperature measured by thermometer 31 is applied via a lead, 32, to processor 28 which produces a control signal on a lead, 33, for controlling the temperature of heater 30.
Thereafter, fiber 20 enters a variable length, or telescoping, carbon-coating chamber 34, wherein a mixture of acetylene precursor gas together with chlorine and an inert gas, such as nitrogen, argon, or helium, is applied to the hot surface of moving fiber 20 for inducing decomposition of the acetylene precursor gas and deposition of a carbon coating uniformly on the periphery of the fiber. Signal indicative of the fiber temperature within chamber 34 is sent via lead 60 to processor 28 which may, if needed, send a signal via lead 33 to heater 30 for changing the temperature, or via lead 58 for changing the length of the chamber 34, or both. Signal indicative of the gas pressure in chamber 34 is transmitted via a lead, 62, to processor 28 which may send a control signal for changing the pressure via lead 56 to pressure regulator, 57. An indication of the concentration of the acetylene gas is forwarded from chamber 34 via a lead, 63, to processor 28. A control signal for changing the mixture of gases is transmitted from processor 28 via a lead, 64, to gas supply valves 65, 66 and 67. The gases are mixed in a manifold, 68, and delivered through pressure regulator 57 and supply line 40 to gas chamber 34. The gases exit gas chamber 34 by way of an exhaust fitting, 45.
The carbon coated fiber, upon exiting carbon-coating chamber 34, moves through a radio frequency resonator 35. The thickness of the carbon coating on the surface of the moving optical fiber 20 is measured in resonator 35 without physically contacting the unjacketed fiber.
Upon exiting from resonator 35, carbon coated fiber 20 moves on through a fiber jacketing stage. This stage includes one or more vessels, 46, filled with ultraviolet light curable liquid materials, and a set of lamps, 47, applying ultraviolet light for curing the liquid material on the fiber. The jacketed fiber is wound about a capstan drive, 48, and then onto reel 49 for storage. Signals indicative of the speed of capstan 48 and jacketed fiber 20 are sent to processor 28 via a lead, 61.
The carbon-coating thickness measurements, as disclosed in the above-mentioned Atkins et al. patents, are conducted utilizing resonator 35 which may have various configurations. In each of these variants the resonator includes a solid wall outside chamber, a radio frequency electromagnetic field is established within the resonator and the coated fiber passes through the resonator disturbing the field. The electromagnetic field is established in resonator 35 by an input signal from a source of radio frequency oscillations, 41. As the carbon-coated insulator is moved through the energized electromagnetic field, interaction of the conductive coating with the electromagnetic field or a component thereof induces transmission loss from input to output. An output signal is extracted from the electromagnetic field at a point where the output signal can be detected. The effective radio frequency conductance of the carbon coating is then determined from changes in the output signal with respect to a predetermined standard. Thickness of the coating is determined from the conductance data. From the thickness determination, signals are generated for dynamically controlling the coating process to maintain a desired thickness tolerance.
In FIG. 4 is shown a schematic representation of one version of prior art resonator 35 for operation in the radio frequency range. The resonator includes a cylindrical metal chamber, 36, and a conductive helical coil, 37, within the chamber. The coil is affixed at one end, 38, to chamber 36 and otherwise is separated from the walls of the chamber. An input signal, produced by fixed or swept frequency signal generator 41, is coupled into the resonator via coaxial line 42 by an input coupling loop 43. Input loop 43 is positioned inside of the resonator to energize a resonant electromagnetic field in response to the applied input signal. The frequency at which the helix is operable may be adjusted by means of a capacitor (not shown) connected between coaxial line 42 and coil 37. Power levels of the input signal typically can be in the range of from a fraction of a milliwatt to about 100 milliwatts. By energizing coil 37 with a radio frequency input signal, an electromagnetic field is established within and along coil 37. The introduction of a coated dielectric (a carbon coated optical fiber) into the resonator axially of coil 37 modifies both the resonance frequency and output power. As the carbon coated fiber 20 moves through the electromagnetic field, it absorbs power from it. Presence of coil 37 in chamber 36 tends to increase the electromagnetic field along the axis of the chamber, which coincides with the direction of movement of fiber 20 through the chamber. Power is extracted from the resonator by way of an output coupling loop, 50, positioned so that it interacts with the appropriate field where the field strength is sufficient to produce a useful output signal. The output signal extracted from the resonator is transmitted via coaxial line 52 to processor 28. The transmission response is related to conductance of the carbon coating. Processor 28 determines the thickness of the conductive carbon coating and, if needed, develops a signal for controlling the carbon deposition process.
A variant of a prior art resonator for measuring the thickness of a coating on an optical fiber is described by J. Y. Boniort et al. "New Characterization Techniques for Hermetic Carbon Coated Fibers", ECOC-100C 91, Paris, France. Shown in FIG. 5 is a resonator 70 having a thin elongated helical coil, 71, with a small central aperture held between two metallic end plates, 73 and 74, within a metallic outer cylinder, 72, providing an electromagnetic shield. The coil is connected at both ends to the metallic end plates. Input and output terminals, 75 and 76, respectively, are located at opposite ends of the coil. A radio frequency signal, is provided to input terminal 75 via line 77. The output signal is extracted from the resonator via output terminal 76, and could be sent via coaxial line 78 to a processor, such as processor 28 shown in FIG. 3. In the Boniort et al. system most of the electromagnetic energy is contained outside of the helix, that is between the helix and the outer cylinder. In this arrangement the oscillator signal source is not locked to the resonant transmission peak of the helix but is simply swept through the resonator. This makes a closed loop control system difficult to implement.
The measuring arrangements described by Atkins et al. and by Boniort et al. present certain other problems. One of the problems is that the resonator is opaque so that the fiber cannot be viewed. Furthermore, the opening through which the fiber passes is very small, e.g., 0.1-0.3 cm in diameter and typically only 0.1 cm. This makes fiber insertion difficult. The small size of the opening also increases the possibility of the fiber touching the walls of the opening during the manufacturing. Should the unprotected fiber rub against the walls of the opening, the fiber can be damaged or even break. Considering the diameter of the carbon-coated fiber and the fact that a length of the optical fiber between the preform and the jacketing stage is subject to oscillations transverse to the movement of the fiber along its longitudinal axis, such contact is quite likely.